1. Field of the Invention
The present invention relates to a fixing device for causing a pressurizing roller to come in pressure contact with a heating roller including a central region heating heater (first heater) for heating a central region and a side end region heating heater (second heater) for heating a side end region on the outside of the central region, thereby fixing an unfixed developer, and an image forming apparatus comprising the fixing device.
2. Related Art
As a fixing device of an image forming apparatus, in a fixing device including two heaters using two halogen lamps for a heating roller (see Japanese Patent Publication No. JP-A-8-123230, for example), a first heater having a greater calorific value in the central part of the heating roller than that on both side ends and a second heater having a greater calorific value on both side ends of the heating roller than that in the central part are combined to cause a temperature distribution to be uniform corresponding to a passing paper size.
In the fixing device having the two heaters, accordingly, a region in which the halogen lamp is to emit a light and a region in which non-light emission is to be carried out are present in each of the heaters. More specifically, the first heater has a central region in which the light emission is to be carried out and a side end region in which the non-light emission is to be carried out, and the second heater has a central region in which the non-light emission is to be carried out and a side end region in which the light emission is to be carried out. Corresponding to the regions, a coil-shaped filament (hereinafter referred to as a coil filament) is provided in the region in which the light emission is to be carried out, and a wire (hereinafter referred to as a wire filament) is provided in the region in which the non-light emission is to be carried out. Furthermore, the coil filament and the wire filament are to be alternately provided in the region in which the light emission is to be carried out, and a coil-shaped holding portion filament is to be provided in order to eliminate the slack of the wire in the region in which the non-light emission is to be carried out.
However, the holding portion filament emits a light so that a light distribution cannot be set to be zero %. In the conventional device (JP-A-8-123230), therefore, light emission and heat generation which are to be carried out by the holding portion filament present in the central region of the second heater are decreased to 5 to 20% as a whole non-light emitting region so as not to reduce the efficiency of the heater.
However, the holding portion filament is to be provided in the side end regions of the first heater in addition to the central region of the second heater. Also in the side end regions of the first heater, the light distribution of the holding portion filament is present. For this reason, in the case in which the lighting duty of the first heater is high (in case of warm-up and a thick and continuous paper), the temperature of the heating roller in a portion corresponding to a light distribution peak is raised. As a result, if the light distribution peak is present in an image region, there is a problem in that a gloss unevenness is generated on an image or a high temperature offset is generated.
Further, a temperature ripple is generated corresponding to the alternate arrangement of the coil filament and the wire filament in the first heater in the central region as described above, and furthermore, is generated corresponding to the alternate arrangement of the holding portion filament and the wire filament in the second heater. In the case in which the heating roller is heated by using the first heater and the second heater, therefore, these are superposed on each other so that a great temperature ripple is generated. Consequently, there is a problem in that a gloss unevenness is generated on an image in the region in which the non-light emission is to be carried out.
In addition, the temperature ripple generated by the second heater fluctuates due to the ON/OFF operation of the second heater. Consequently, precision in the detection of a temperature sensor for controlling the first heater is deteriorated and a variation in the control of a fixing temperature is increased. Thus, there is a problem in that a gloss unevenness is generated.